Inverters serve to convert a direct current generated by a photovoltaic generator into alternating current which can be fed as single- or multi-phase current into a public or private power supply grid. A photovoltaic generator, hereafter referred to as a PV generator, should be understood in the context of the application as being any arrangement of photovoltaic modules (PV modules), in particular an arrangement in which multiple PV modules are connected in series to form a so-called string. In photovoltaic installations (PV installations) with increasing output, this type of connection becomes more important to minimize ohmic losses in the direct current lines between the PV generator and the inverter.
Inverters have (buffer) capacitors located in their direct current input circuit or in a direct voltage intermediate circuit to smooth the direct voltage despite the pulsed electricity consumption which results during the conversion into alternating current, and to increase the maximum peak current which can be drawn off in a pulse. However, the capacitors prove to be problematic when connecting a PV installation to the power supply grid as in some circumstances unacceptable high charging currents may flow from the power supply grid into the capacitors.
To prevent high charging currents when the PV installation is connected to the power supply grid, the capacitors are charged beforehand to a suitable bias voltage before the PV installation is connected to the power supply grid. As for example known from the document DE 197 35 867 A1, a separate pre-charging device may be provided for this purpose, for example implemented as a connectable pre-charging transformer. However, this entails additional structural complexity.
It is also known to pre-charge the capacitors via the PV generator. PV generators are usually operated by the inverter, with the aid of a tracking system, the so-called MPP (maximum power point) tracker, at a working point at which the maximum electrical power is output. The working voltage at this working point is considerably smaller than the open circuit voltage of the PV generator. For cost reasons, the electric strength of the semiconductor components in an inverter, in particular the power semiconductors in an inverter bridge of the inverter, is usually not designed for frequent or permanent operation with an open circuit voltage. A regular pre-charging of the capacitors in the inverter to the open circuit voltage of the PV generator could thus have a negative effect on the life-time of the semiconductors in the inverter.
It is known from the document JP 113 12 022A to protect the inverter bridge of an inverter by a resistance voltage divider in the direct current input circuit from the high open circuit voltage of a PV generator when the capacitors are pre-charged. During operation, the resistance voltage divider is removed from the connection between the PV generator and the inverter by corresponding switching elements. However, both the resistance voltage divider and the means for bridging it represent greater structural complexity.
It is therefore desirable to provide a method for connecting a PV installation having an inverter to a power supply grid. The method allows the PV installation to be connected with little additional structural complexity and without the inverter being loaded with unacceptably high voltages or currents.
According to one aspect of the disclosure, a method for connecting a photovoltaic installation to a power supply grid is provided. The photovoltaic installation comprises a photovoltaic generator, a direct voltage intermediate circuit with at least one capacitor, and an inverter. The direct voltage intermediate circuit is first connected to the photovoltaic generator and the at least one capacitor is pre-charged to a first voltage. The direct voltage intermediate circuit is then separated from the photovoltaic generator. The at least one capacitor is then discharged to or below a second voltage that corresponds to a maximum operating voltage of the inverter. The inverter is then connected to the power supply grid, an inverter bridge of the inverter is clocked, and the direct voltage intermediate circuit is connected to the photovoltaic generator.
In the above manner, the at least one capacitor in the direct voltage intermediate circuit is first pre-charged via the photovoltaic generator without the inverter being loaded with a high voltage of the photovoltaic generator, for example, its open circuit voltage. When a voltage which does not harm the inverter is reached by discharging the capacitor to or below the second voltage, the inverter is connected to the power supply grid and the inverter bridge of the inverter starts to clock. Neither unacceptably high voltages nor unacceptably high currents occur at the inverter during the pre-charging, which voltages or currents could harm the inverter.
According to an other aspect, an alternative method comprises similar acts with a difference that connecting the inverter to the power supply grid is performed before separating the direct voltage intermediate circuit from the photovoltaic generator. The same advantages as described with the first aspect arise.
In further embodiments of the methods according to the above mentioned aspects, the act in which the direct voltage intermediate circuit is separated from the photovoltaic generator only takes place when the first voltage is greater than a rectifying voltage of the power supply grid. It is thereby prevented that charging currents flow via the inverter into the capacitor of the direct voltage intermediate circuit when the inverter is connected to the power grid. In a further embodiment, the act in which the inverter is connected to the power supply grid takes place when the first voltage is essentially equal to an open circuit voltage of the photovoltaic generator. This way, the criterion mentioned beforehand according to which the first voltage should be greater than the rectifying voltage can easily be fulfilled simply providing an appropriate time during which the direct voltage intermediate circuit is connected to the photovoltaic generator. A voltage that is essentially the same as the open circuit voltage of the photovoltaic generator is e.g. a voltage higher than approximately 90% of the open circuit voltage.
In further embodiments of the methods, a nominal voltage in the direct voltage intermediate circuit is set by clocking the inverter bridge before the direct voltage intermediate circuit is connected to the photovoltaic generator. That way the inverter can be used to control the voltage of the intermediate circuit to an advantageous level, e.g. to a voltage close to an expected working voltage.
In further embodiments of the methods, the at least one capacitor is discharged to or below the second voltage by a resistor connected in parallel to the at least one capacitor. This provides a simple discharging method for the at least one capacitor.
According to another aspect, a photovoltaic installation is provided that has a photovoltaic generator which is connected to an inverter via at least one direct current switching element and a direct voltage intermediate circuit. The direct voltage intermediate circuit here has a capacitor. The inverter can be connected to a power supply grid via an alternating current switching element. The photovoltaic installation comprises a control device for controlling the inverter, the direct current switching element and the alternating current switching element, wherein the control device is configured so as to carry out the above-described method. Again, the same advantages as described above arise.